1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device containing an orientation film having one or more charge transporting materials (hereinafter CTM) doped therein, and thus no crosstalk occurs due to the formation of an electric bilayer in the liquid crystal medium of the liquid crystal (hereinafter LC) device. Since no crosstalk occurs, in addition to the general advantages of the LC device itself such as a reduced area of installation, less eyestrain and lower consumption of electric power, the LC device of the present invention can be widely used as a display element or panel in electric devices, i.e., office automation (hereinafter OA) devices such as word processors, lap top and personal computers, and work stations, etc. Especially, when a super twisted nematic (hereinafter STN) liquid crystal is used in the LC device, the device of the present invention will provide additional advantages such as a large-scale and high information content display, high contrast and high gradation display.
2. Description of the Related Art
Nowadays, many types of liquid crystal display (hereinafter LCD) devices are widely utilized as a display element in the OA devices. One such display device is a twisted nematic (hereinafter TN) liquid crystal display device which is now widely used in, for example, watches and electronic calculators. The TN-LCD device is generally driven at a high duty ratio of up to 1/100, but suffers from a defect in that the display area is gradually darkened or lightened with an increase of the display lines, and thus the characters and other information on the display area become difficult to read. This display defect is considered to be due to a poor electro-optical steepness of the TN liquid crystal, and is referred herein to as crosstalk of the first kind.
Recently, a new type of the liquid crystal display device, i.e., the super twisted nematic (STN) liquid display device, has been developed, and is now widely used in OA devices because of its excellent multiplexability. The dramatic improvement of the multiplexability of the STN-LC device solves the above-discussed problem of the crosstalk of the first kind, because the steepness in the electro-optical characteristic of the LC device is improved. Nevertheless, in addition to this effect, another kind of crosstalk, which will be referred herein to as crosstalk of the second kind, is sometimes observed in a highly multiplexed STN-LC device. The crosstalk of the second kind is particularly remarkable when specific voltage waveforms are applied to particular pixels. This second kind of crosstalk frequently appears when a line image is displayed or a highly symmetric fine image is displayed, and it is sometimes hard to recognize the displayed image. For example, solid lines in graphs or tables are displayed with a ghost image thereof extending from the end point of the solid lines. Further, in the case of Chinese characters (kanji): in the character " " (Fu of Fujitsu Limited), for example, the lower portion " " of Fu is not clearly displayed but appears as -- (black box)--. Furthermore, if a drive voltage is controlled to exactly display the above character, the upper portion " " of Fu is then changed to -- --. As will be described hereinafter, the second kind of crosstalk is considered to be due to a shifting of the transmittance (T)-drive or applied voltage (V) curve in the LC device. This crosstalk is unavoidable, even if the applied voltage has a relatively low value, because the transmittance is largely varied as a function of a shifting of the T-V curve. Since the second crosstalk is a critical problem for the STN-LC devices, there is a need for the provision of an improved STN-LC device not having this crosstalk problem.
In the present invention, the shifting of the T-V curve, and accordingly, the generation of the second crosstalk, can be prevented by using an orientation film and at least one charge transporting material doped therein. Nevertheless, there is no prior art teaching or disclosing the use of the charge transporting material-doped orientation film for the purpose of eliminating the second kind of crosstalk in the LC devices, and particularly, STN-LC devices, although some prior art teach the incorporation of additive(s) to an orientation film of the LC devices but for a different purposes from that of the present invention. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-295028 concerns the improvement of an orientation film in the LC device, and teaches the formation of the orientation film from a polyimide having a specific structure, or a precursor thereof, and metal powders and/or electrically conductive organic compounds such as NMP-TCNQ, TTF-TCNQ, and charge transfer complexes. The resulting low resistance film effectively prevents an accumulation of electric charges due to an unevenness of the thickness of the orientation film and a change in color of color filters used in color LC devices. Further, Japanese Unexamined Patent Publication (Kokai) No. 63-121020 also concerns the improvement of the orientation film in the LC device, especially a ferroelectric LC device, and teaches a control of a conductivity of at least one orientation film to 1.times.10.sup.8 .OMEGA.cm or less, by using 5% by weight or more of a conductive material such as carbon or metal particles (Au, Aq) in a resinous orientation film. The resinous orientation film effectively prevents a notable reduction of the display characteristics of the FLC device due to ionic contaminants generated in the LC layer as a function of the polarization charges of the LC molecules.
On the other hand, Kenji Nakaya et al., of Tokyo University of Agriculture and Technology teach in Japanese Journal of Applied Physics, Vol. 28, No. 1, 116 (1989) that the incorporation of charge transfer complexes in an orientation film of the FLC device effectively prevents an accumulation of electric charge in an area adjacent to the orientation film. Nevertheless, the charge transfer complexes have drawbacks such that they are chemically unstable and sensitive to water, and therefore, cannot be used in a polyimide, which is a typical orientation film material, and further, are liable to be changes with an elapse of time. The orientation film of the above prior art exhibits low electric resistivity under direct current electric field, so, the orientation film sacrifices the electrically insulating characteristic of the transparent electrode. On the other hand, the orientation film of the present invention which will be described hereinafter exhibits insulating characteristic under direct current electric field and electric conductive characteristic as a result of hopping conductivity under high frequency electric field such as pulse writing frequency. So, the orientation film does not sacrifice the insulating characteristic of the transparent electrode and can prevent the temporary accumulation of the electric charge.